How to change dimensions of an object proportionally?
There is a solution since version 2.70:
http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.70/UI#Multi-Number_Button_Editing

Click and drag from the first to the last fields you want to edit and release the click. The three fields are highlighted. Enter an expression, eg. *3/2 and press enter. All fields are edited at the same time.